Six Inch Cross Fusor

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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

Update: smooth run today conditioning the grid at 42 kV and 2 mA. I am very happy with the control characteristics of the system. I am able to control steady current to the tenth of a mA and plasma holds at 0.8 mA without extinguishing.

My method for eliminating corona on the external side seems to be working okay for now. I coated the connection with hysol 1c epoxy. I added some teflon after that.

My neutron numbers are relatively low right now but I'm sure they will improve with loading.
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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

More progress. 43 kV and 8 mA in this evenings run. My Russian tube, which read 600 cpm last weekend reached 2000 cpm. I am pretty sure my feedthrough has the ability to manage more.

I need to get a decent silver foil and try some activation. What thickness foil do people recommend?

Jim K
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Mark Rowley
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Re: Six Inch Cross Fusor

Post by Mark Rowley »

Congrats Jim!
For this I don’t think you are limited to foil. A Sterling spoon, Mercury Dime, ingot, etc will be fine. Zap it for as long as you can and quickly plop it down on a pancake.

MR
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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

Maybe I should dig out my atomic energy museum dime and activate that. 😁
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Richard Hull
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Re: Six Inch Cross Fusor

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The only reason foil is used at all is that thickness is just a waste if you are using a GM counter as it is sensitive to Betas for the most part and in metal, betas might penetrate .001-inch to get out of a silver spoon or coin which is only 92.5% Ag. Whatever you activate must be a good beta emitter for GM activation detection.

If you have a good gamma spectrometer and the element you are activating has a decent half-life and is mostly a good gamma emitter. Thick can be used to advantage up to a point depending on the energies of the gammas. Naturally, the Gamma spec will not see any Betas.

It is a matter of knowing your quarry intimately in any activation effort. All of this demands some fore-knowledge from a good isotope data table. I have posted on the ideal "Table of the Isotopes" book where useful data can be had prior to activation of any element on the planet. I sold 4 of my 8 copies of the Table of the Isotopes at HEAS 2020 in October. They were snapped up in less than a few hours during the large flea market.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

Richard,
You are dead on about activation. Having spent a bunch of my youthful formative time doing activation analysis using the UVAR not too far from your abode, I own several booklet copies of the chart of the nuclides. I haven't yet mounted it on the wall of my South Carolina lab, but I also have a full size poster. I also spent more time with the barn book than I care to repeat (I wish I had one now though). I also have here a decent activation analysis text.

All of that was about gammas though, and as you pointed out, fledgling fusor activation can be best demonstrated by not ignoring the beta emission of activated silver. My original question about foil thickness was for this purpose, and I was hoping someone would share what thickness worked best for them. It's all about a balance of enough target mass while avoiding self shielding. From my own quick research, I'm thinking 0.3 mm will do for silver. I think I will look for that unless someone suggests a different thickness based on their experience.

Mark has a good point about just using an easily available silver object. After all, activating something like that demonstrates that neutrons can be used to identify isotopes within materials we have reason to be curious about. I may start with this just for practicality.

I am also designing a demonstration cell based on your foil wrapped GM. I have on hand a small, beta sensitive, thin window tube that I would face with silver and then encapsulate it within some hdpe beads. Ultimately, I envision extracting pulses that I would feed through a PC sound card into PRA software which has a feature of plotting counts vs time. If I get to this, I will start a new thread.

Finally, to finish off this post, I offer this picture which I ripped off from an Ortec lab experiment available multiple places on the web. It is a great chart of neutron activation sensitivities that can be used to pick out candidates for more than fledgling fusor activation studies.
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Jim Kovalchick
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Re: Six Inch Cross Fusor

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Update:
My fusor hit a new neutron milestone tonight. Steady state 12,000 cpm on my Russian tube with brief excursions during some adjustments well over 20,000.

I placed a one ounce silver coin between 1.5 inches of hdpe on each side. After a quick shutdown of the fusor lasting 10 to 15 seconds, I placed the coin directly on a 2 inch pancake GM and saw 120 cpm. Background is 30 to 40 cpm. I have some foil coming and should see better results then.

Jim K
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

30,000 cpm on my Russian tube. 42 kV 13.5 mA.

I activated a piece of 30 gage sterling silver sheet to 5 times background. It probably would have been more if I had shutdown and measured when my counts first started to die when my chamber got too hot.
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Re: Six Inch Cross Fusor

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Good going Jim! You obtained about the exact same activation level that I got on silver at that exact voltage and current during and after HEAS 2020. Glad you are back doing good fusion. We just have to water cool in future, no matter what the chamber design if we are to increase the fusion numbers pushing the systems.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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Re: Six Inch Cross Fusor

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A little more fusor eye candy from today. -44 kV and 13.5 mA.
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

I have not been satisfied with progression of my neutron numbers. In fact, they have gone down. I replaced my grid with a beefy 304 stainless tube that proportionally better matches Jon Rosenstiel's to see what would result.

Neutron numbers stay low with the new grid until the grid gets red. For the same voltage and current, there is almost a ten fold increase in numbers once the grid is glowing hot. I assume this change is from thermionic electrons making plasma more efficiently. I get 30,000 cpm on my Russian tube, but it lasts for only about a minute, because by the time my hefty stainless grid is hot, so are the chamber walls. I think I may experiment with grids that make better use of the work function earlier.
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Richard Hull
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Re: Six Inch Cross Fusor

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I think active cooling is a must, but try going back to a spherical tungsten grid before cooling the cross. Lots of ideas rolling around in my head now with 6 different ways to go from here, (long term).

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

Richard,
I started planning to go back to a tungsten wire grid even before I posted yesterday. I have also been pondering using an unconventional wire spacing. The last time I used a wire grid in the cross, the beams seemed to be influenced by the chamber wall shape, and the beams did not appear focused.

As far as cooling, I have some ideas that will take some time to implement, but in the mean time I have added a second muffin fan.
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Richard Hull
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Re: Six Inch Cross Fusor

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Jim you are correct the cross is not a great beaming device save for along its axis. a spherical grid will tend to field line along the sharp edges in the cross arms as they are all quite close to the grid compared to the flat ends. Those edges, even if carefully rounded or smoothed/polished are still high field regions and tend to force any beaming solely due to proximity. Naturally it all works to do fusion, the cube is nice solely due to target, (flat end), proximity to the cathode grid, especially due to any target added to and protruding from the near zero distant cathode. (no need to travel down a metal tunnel to get to the more distant target, as in a cross)

The original Farnsworth all spherical system allows for tight field control better than any other form. A short cylinder with a tubular hollow grid is good as is the cube with short cathode to target distances. These latter forms are more beam on target.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Jim Kovalchick
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Re: Six Inch Cross Fusor

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Here is a picture of my latest grid. You can see where the secondary beams form.
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Richard Hull
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Re: Six Inch Cross Fusor

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Yes, the smoth round ends will not beam against the sharp edges of the cross but yield a rather diffuse coronal glow as the field off the ends is rather evenly distributed about the ends and the equally uniform sharp cross edges. (They share the "glow current" rather smoothly, robbing power from the central beaming) . Only the central beam lines are true hard, visible beams.

Question is there a diamond lozenge on the other side of that cathode?? If not, one of the side arms has a metallic protrusion that is beaming to that area. Screen wire in one of the side arms to protect a view port would probably do this.

Richard Hull
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Fusion is the energy of the future....and it always will be
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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

Richard,
There are other beam marks on the grid. There is an almost identical one on the other side, a smaller one opposite my connecting wires, and a smaller one yet near my connecting wires.

The secondary beams seem weak. The one that made the mark you see in the picture goes to my view port. The view port has had no visible damage and when operating has a wide blue fluorescence glow.

Jim K
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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

Some updates:

I have installed a tungsten wire grid in replacement for the SS tube grid. I spaced the wires to leave wider openings to the flat sides of my cross. This seems to have forced most of the heat to one of three of the wires. This grid runs at lower pressures and as a result, at the voltage I ran it, the beams can't really be seen over the light of the tungsten incandescence.

As far as relative neutron production, I can't say. Yesterday, my Russian tube decided it had had enough and failed. Today I used a large, old Nancy Wood BF3 tube that I am still tweaking. It is a little sentiment to noise at the low end but while running it seemed to give real data. I really need to do a moderator test just to be sure, but it was too close to my feedthrough to safely fiddle with. I could see fusion rates improve across the run so I don't think that I was watching xrays. For point of new reference, I saw 60,000 cpm at 40 kV and 7 mA.

As far as temperature, I did NOT see any signs of chambet temperature degrading my neutron numbers. I ran for 5 minutes at 60,000 cpm without seeing numbers sag like they did before with my ring grids. I didn't record the numbers this time but my silver activation cell gave counts right after the run that looked as good or better than counts in saw at higher voltage and currents with ring/tube grids.

Interesting data. More tests coming.
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Re: Six Inch Cross Fusor

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Spherical symmetry is the ideal, but cube and correctly constructed cylinders seem to take advantage of beaming. This is especially true with active cooling in the smaller sized cubes. The cubes do so well due to shorter beam distance (MFP?) and the intrinsic non prominence of the sharp edges found in a long throw cross. Cubes are nothing more than a cross with zero arm length. Cylinder fusors would benefit from large diameter, short length. A 6"X6" cylinder with a direct mounted 2.75 CF HV insulator in the center using a cylinder cathode of a length that would have a shorter distance to the two target ends than from the cylinder cathode body to the chamber cylinder walls. This might be a good performer. The cylinder would be a snap to cool with coiled Cu tubing wound around the cylinder.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Six Inch Cross Fusor

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Based on my silver activation cell data I seem to be getting much better numbers with the wire grid than my tube and ring grids. I am using less d for the same voltage and current. Even general area xray dose rates are lower. The first plot is from the ring grid. The second is from today using the wire grid.
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Richard Hull
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Re: Six Inch Cross Fusor

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Yes, a good deal better and much better looking decay curve. Glad it is at least working better for you.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

Posting this today just to share a little eye candy. Because I've been testing out my new heavy water conversion, I did several runs today. The wall loading has been fantastic. I fired up an old Eberline PNC and sitting in my lap six feet from the fusor it read 250 cpm. I was running at 45 kV and 11 mA.
.
My picture was probably closer to 30 to 35 kV, but I like the colors. Most of the color was courtesy of grid incandescence, but there some plasma pink too.

Jim K
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Re: Six Inch Cross Fusor

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Great result Jim! It looks as if the wire grid is doing the work for you now.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Jim Kovalchick
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Re: Six Inch Cross Fusor

Post by Jim Kovalchick »

I have been looking for ways to reduce noise in my system. I know I must be getting some from my feedthrough connection based on hissing and snapping sounds.

This latest feedthrough iteration is my take on oil potting. It's still a prototype, and I'm still working on the seal. I am procuring a nylon pipe clamp that should help cure the tiny weep I'm getting now.

Jim K
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Dan Knapp
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Re: Six Inch Cross Fusor

Post by Dan Knapp »

Jim, a suggestion regarding oil potting. Something I’ve been planning to try is using Vaseline in place of oil. Warm it enough to pour it in then let it solidify. Yes, heat of operation is likely to liquefy it, but a slow leak during periods of inactivity would be less likely to make a mess.
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